Process for the production of a multicoat finish and aqueous basecoat suitable for this process

ABSTRACT

The invention relates to basecoats for the production of paint systems of the basecoat-clearcoat type, which comprise a water-thinnable polyacrylate resin obtainable by polymerizing in a first stage in an organic solvent ethylenically unsaturated monomers free from carboxyl groups and, after at least 80% by weight of these monomers have reacted, polymerizing in a second stage monomers containing carboxyl groups and neutralizing the polyacrylate resin obtained in this way.

This is a continuation of application Ser. No. 07/927,522 filed on Sep.17, 1992, now abandoned.

The invention relates to a process for the production of a multicoat,protective and/or decorative finish, in which process

(1) an aqueous pigmented basecoat is applied to the substrate surface asbasecoat,

(2) a polymer film is formed from the coating applied in stage (1),

(3) a transparent topcoat is applied to the basecoat obtained in thisway, and subsequently

(4) the basecoat is baked together with the topcoat.

This process represents the well known basecoat/clearcoat process whichis employed especially in the automotive industry for the production ofhigh-quality finishes, in particular metallic effect finishes (cf. forexample EP-A-38,127, EP-A-89,497 and DE-A-3,628,124).

The invention also relates to aqueous paints which can be used asbasecoats in the above process.

The basecoat/clearcoat process under discussion employs predominantlybasecoats which contain exclusively organic solvents as thinners and/orsolvents.

The coatings industry has striven for ecological and economic reasons toreplace as large a part of the organic solvents as possible by water.There is a great demand for aqueous basecoats which can be used in thebasecoat/clearcoat process described above. An essential feature of thebasecoat/clearcoat process is that the transparent topcoat is applied tothe unbaked basecoat, and only then are the basecoat and topcoat bakedtogether (wet-on-wet process).

The object of the present invention consists in providing novel aqueouspaints which can be used as basecoats in the basecoat/clearcoat process.Surprisingly, this object is achieved by the provision of aqueous,pigmented paints containing a water-thinnable polyacrylate resin whichcan be obtained

(I) by adding

(a1) 40 to 90, preferably 40 to 80, % by weight of a (meth)acrylic acidester essentially free from carboxyl groups, or a mixture of such(meth)acrylic acid esters, (a2) 0 to 45, preferably 4 to 34, % by weightof an ethylenically unsaturated monomer which contains at least onehydroxyl group per molecule and is essentially free from carboxylgroups, or a mixture of such monomers, and

(a3) 0 to 40, preferably 10 to 30, % by weight of an ethylenicallyunsaturated monomer essentially free from carboxyl groups, which isdifferent from (a1) and (a2), or a mixture of such monomers,

to an organic solvent or mixture of solvents and carrying out apolymerization in the presence of at least one polymerization initiator,and

(II) by adding, after at least 80% by weight of the monomers added instage (I) have reacted,

(b1) 2.5 to 15, preferably 3 to 7, % by weight of an ethylenicallyunsaturated monomer containing at least one carboxyl group per molecule,or a mixture of such monomers, and

(b2) 0 to 60, preferably 0 to 28, % by weight of an ethylenicallyunsaturated monomer essentially free from carboxyl groups, or a mixtureof such monomers,

and continuing the polymerization and

(III) by neutralizing, at least partially, the polyacrylate resinobtained at the end of the polymerization and dispersing it in water,the sum of the proportions by weight of (a1), (a2), (a3), (b1) and (b2)always 100% by weight and the type and amount of (a1), (a2), (a3), (b1)and (b2) being chosen so that the polyacrylate resin obtained from (a1),(a2), (a3), (b1) and (b2) has a hydroxyl value of 0 to 200, preferably20 to 120, an acid value of 20 to 100, preferably 25 to 50, and a glasstransition temperature (T_(G)) of -40° to +60° C., preferably -20° to+40° C.

The water-thinnable polyacrylate resins used according to the inventionallow the formulation of basecoats which possess--especially incomparison with known basecoats containing polyacrylate resins--a highersolids content and a lower tendency to form runs. In addition, the useof the water-thinnable polyacrylate resins according to the invention,especially in polyurethane-containing basecoats, provides stabilizationagainst fluctuating shearing stresses. Compared with the state of theart, a lower reduction in viscosity, or no reduction at all, occursespecially on exposure to weak shearing forces. This results in improvedsedimentation behaviour, simpler handling and increased applicationreliability.

A more detailed description of the preparation of the water-thinnablepolyacrylate resins essential to the invention is preceded by anexplanation of two terms used:

1) The term "(meth)acrylic acid" is occasionally used as an abbreviationfor "methacrylic acid or acrylic acid".

2) The expression "essentially free from carboxyl groups" is intended toindicate that the components (a1), (a2), (a3) and (b2) may have a lowcarboxyl group content (but no higher than would cause polyacrylateresin prepared from the components (a1), (a2), (a3) and (b2) to have anacid value no higher than 10). It is preferred, however, for thecarboxyl group content of the components (a1), (a2), (a3) and (b2) to beas low as possible. Components (a1), (a2), (a3) and (b2) which areentirely free from carboxyl groups are particularly preferred.

For the preparation of the polyacrylate resins to be used according tothe invention any ester of (meth)acrylic acid which is essentially freefrom carboxyl groups and is copolymerizable with (a2), (a3), (b1) and(b2), or a mixture of such (meth)acrylic acid esters, may be used as thecomponent (a1). Suitable examples are alkyl acrylates and alkylmethacrylates having up to 20 carbon atoms in the alkyl radical, such asmethyl, ethyl, propyl, butyl, hexyl, ethylhexyl, stearyl and laurylacrylate and methacrylate, and cycloaliphatic (meth)acrylic acid esters,such as cyclohexyl (meth)acrylate. Mixtures of alkyl acrylates and/oralkyl methacrylates which consist of at least 25% by weight of n-butyland/or t-butyl acrylate and/or n-butyl and/or t-butyl methyacrylate[sic], are preferably used as the (a1) component.

Any ethylenically unsaturated monomer which contains at least onehydroxyl group per molecule, is essentially free from carboxyl groupsand is copolymerizable with (a1), (a3), (b1) and (b2), or a mixture ofsuch monomers, may be used as the component (a2). Suitable examples arehydroxyalkyl acrylates, hydroxyalkyl methacrylates or hydroxyalkylesters of another α,β-ethylenically unsaturated carboxylic acid. Theseesters may be derived from an alkylene glycol esterified with the acid,or they may be obtained by reaction of the acid with an alkylene oxide.Hydroxyalkyl acrylates and hydroxyalkyl methacrylates in which thehydroxyalkyl group contains up to 4 carbon atoms, reaction products fromcyclic esters, for example ε-caprolactone, and these hydroxyalkylesters, or mixtures of these hydroxyalkyl esters orε-caprolactone-modified hydroxyalkyl esters, are preferably used as thecomponent (a2). Suitable examples of such hydroxyalkyl esters are2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 3-hydroxypropylacrylate,2-hydroxypropyl methacrylate, 3-hydroxypropyl methacrylate,2-hydroxyethyl methacrylate, 4-hydroxybutyl acrylate and 4-hydroxybutylmethacrylate. Corresponding esters of other unsaturated acids, forexample ethacrylic acid, crotonic acid and similar acids having up toabout 6 carbon atoms per molecule, may also be used.

Any ethylenically unsaturated monomer which is essentially free fromcarboxyl groups, is different from (a1) and (a2) and is copolymerizablewith (a1), (a2), (b1) and (b2), or a mixture of such monomers, may beused as the component (a3). Vinylaromatic hydrocarbons, such as styrene,α-alkylstyrene and vinyltoluene, are preferably used as the component(a3).

The polyacrylate resins used according to the invention are prepared bypolymerizing in stage (I) the component (a1) together with (a2), ifappropriate, and together with (a3), if appropriate, in an organicsolvent or mixture of solvents in the presence of at least onepolymerization initiator.

Those solvents and polymerization initiators which are conventional forthe preparation of polyacrylate resins and suitable for the preparationof aqueous dispersions may be used as the organic solvents andpolymerization initiators. Examples of usable solvents are butyl glycol,2-methoxypropanol, n-butanol, methoxybutanol, n-propanol, ethyleneglycol monomethyl ether, ethylene glycol monoethyl ether, ethyleneglycol monobutyl ether, diethylene glycol monomethyl ether, diethyleneglycol monoethyl ether, diethylene glycol diiethyl [sic] ether,diethylene glycol monobutyl ether and 3-methyl-3-methoxybutanol.Examples of usable polymerization initiators are initiators forming freeradicals, such as benzoyl peroxide, azobisisobutyronitrile, t-butylperethylhexanoate and t-butyl perbenzoate. The polymerization isexpediently performed at a temperature of 80° to 160° C., preferably110° to 160° C.

After at least 80% by weight, preferably at least 90% by weight, of themonomers added in stage (I) have reacted, in stage (II)

(b1) 2.5 to 15, preferably 3 to 7, % by weight of an ethylenicallyunsaturated monomer containing at least one carboxyl group per molecule,or a mixture of such monomers, and

(b2) 0 to 60, preferably 0 to 28, % by weight of an ethylenicallyunsaturated monomer essentially free from carboxyl groups, or a mixtureof such monomers are added and polymerized in the presence of thereaction product obtained in stage (I). The polymerization in stage (II)is carried out until the monomers added in stages (I) and (II) haveessentially completely reacted.

Any ethylenically unsaturated monomer which contains at least onecarboxyl group per molecule and is copolymerizable with (a1), (a2), (a3)and (b2), or a mixture of such monomers, may be used as the component(b1). Acrylic acid and/or methacrylic acid are preferably used as thecomponent (b1). However, other ethylenically unsaturated acids having upto 6 carbon atoms in the molecule may also be used. Suitable examples ofsuch acids are etharyl [sic] acid, crotonic acid, maleic acid, fumaricacid and itaconic acid. Mono(meth)acryloyloxyethyl maleate,mono(meth)acryloyloxyethyl succinate and mono(meth)acryloyloxyethylphthalate may also be used as the component (b1).

Any ethylenically unsaturated monomer which is copolymerizable with(a1), (a2), (a3) and (b1), or a mixture of such monomers, may be used asthe component (b2). Any of the monomers listed in the description of thecomponents (a1), (a2) and (a3) may be used as the component (b2).

The type and amount of the components (a1), (a2), (a3), (b1) and (b2)are chosen so that the polyacrylate resin has a hydroxyl value of 0 to200, preferably 20 to 120, an acid value of 20 to 100, preferably 25 to50, and a glass transition temperature (T_(G)) of -40° C. to +60° C.,preferably -20° C. to +40° C.

The glass transition temperatures of polyacrylate resins may becalculated using the following formula: ##EQU1## T_(G) =glass transitiontemperature of the polyacrylate resin x=number of the various monomerscopolymerized in the polyacrylate resin

W_(n) =proportion by weight of the nth monomer

T_(Gn) =glass transition temperature of the homopolymer obtained fromthe nth monomer

The amount and rate of addition of the initiator are preferably chosenso that the resultant polyacrylate resin has a number average molecularweight of 2500 to 20000. It is preferred to commence the addition of theinitiator at the same time as the addition of the monomers and toconclude it about a half-hour after the addition of the monomers hasbeen concluded. The initiator is preferably added at a constant amountper unit of time. When the addition of the initiator is concluded, thereaction mixture is kept at the polymerization temperature until all themonomers used have essentially completely reacted (usually about 11/2hours). The term "essentially completely reacted" is intended toindicate that preferably 100% by weight of the monomers used havereacted, but that it is also possible for a small residual monomercontent of not more than about 0.5% by weight, based on the weight ofthe reaction mixture, to remain unreacted.

When the polymerization is concluded, the resultant polyacrylic resin isat least partially neutralized and dispersed in water.

Both organic bases and inorganic bases may be employed for theneutralization. Primary, secondary and tertiary amines, such asethylamine, propylamine, dimethylamine, dibutylamine, cyclohexylamine,benzylamine, morpholine, piperidine and triethanolamine are preferablyused. Tertiary amines are particularly preferably used as theneutralization agent, in particular dimethylethanolamine, triethylamine,tripropylamine and tributylamine.

The neutralization reaction is generally performed by mixing theneutralizing base with the polyacrylate resin. The amount of base usedfor this purpose is preferably such that the basecoat has a pH of 7-8.5,preferably 7.2 to 7.8.

The partially or completely neutralized polyacrylate resin is thendispersed by adding water. An aqueous polyacrylate resin dispersionresulting therefrom. If convenient, some or all of the organic solventcan be distilled off. The polyacrylate resin dispersions according tothe invention contain polyacrylate resin particles having an averageparticle size preferably between 60 and 300 nm (method of measurement:laser light scattering; measuring instrument: Malvern Autosizer 2C).

Using the polyacrylate resins described above as binders, aqueousbasecoats according to the invention can be produced. However, it ispreferred to combine the polyacrylate resins with at least onewater-thinnable polyurethane resin and/or at least one water-thinnablepolyester resin and/or at least one water-thinnable amino resin asbinders. For basecoats which comprise non-metallic pigments or mixturesof non-metallic pigments only and no metallic pigments, a mixture ispreferably used consisting of

(A) 10 to 95, preferably 25 to 70, % by weight of the water-thinnablepolyacrylate resin according to the invention,

(B) 5 to 50, preferably 10 to 40, % by weight of an amino resin,

(C) 0 to 85, preferably 20 to 60, % by weight of a water-thinnablepolyester resin and

(D) 0 to 85, preferably 0 to 40, % by weight of a water-thinnablepolyurethane resin.

The sum of the proportions by weight the components (A) to (D) is always100% by weight.

For basecoats which comprise a metallic pigment or a mixture of metallicpigments, in combination with non-metallic pigments if appropriate, amixture is used as binder preferably consisting of

(A) 0.1 to 60, preferably 1 to 30, % by weight of the water-thinnablepolyacrylate resin according to the invention,

(B) 0 to 50, preferably 5 to 30, % by weight of an amino resin,

(C) 0 to 50, preferably 15 to 40, % by weight of a water-thinnablepolyester resin and

(D) 10 to 99.9, preferably 20 to 60, % by weight of a water-thinnablepolyurethane resin.

The percentages by weight of the components (A) to (D) always add up to100% by weight.

The polyacrylate resins under discussion may be combined, for example,with water-thinnable polyurethane resins which can be prepared byreacting

(i) a polyester polyol and/or a polyether polyol having a number averagemolecular weight of 400 to 5000 or a mixture of such polyester polyolsand/or polyether polyols,

(ii) a polyisocyanate or a mixture of polyisocyanates,

(iii) a compound which contains in the molecule at least one groupreactive toward isocyanate groups and at least one group capable offorming anions, or a mixture of such compounds,

(iv) if appropriate, an organic compound having a molecular weight of 40to 600 which may contain hydroxyl and/or amino groups, or a mixture ofsuch compounds and,

(v) if appropriate, a compound which contains in the molecule at leastone group reactive toward NCO groups and at least one poly(oxyalkylene)group, or a mixture of such compounds,

with each other and neutralizing, at least partially, the resultantreaction product. The polyurethane resin should expediently have an acidvalue of 10 to 60 and a number average molecular weight of 4000 to25000.

The polyurethane resins may be prepared from (i), (ii), (iii), (iv) ifappropriate and (v) if appropriate by methods of polyurethane chemistrywell known to a person skilled in the art (cf., for example, U.S. Pat.No. 4,719,132, DE-A-3,628,124, EP-A-89,497, EP-A-256,540 and WO87/03829).

Saturated and unsaturated polyester polyols and/or polyether polyols, inparticular polyester diols and/or polyether diols having a numberaverage molecular weight of 400 to 5000 may be used as the component(i). Suitable polyether diols are, for example, polyether diols of thegeneral formula H(--O--)CHR¹)_(n) --)_(m) OH, in which R¹ is hydrogen ora lower, substituted or unsubstituted alkyl radical, n is 2 to 6,preferably 3 to 4 and m is 2 to 100, preferably 5 to 50. Suitableexamples are linear or branched polyether diols such aspoly(oxyethylene) glycols, poly(oxypropylene) glycols andpoly(oxybutylene) glycols. The chosen polyether diols should notintroduce excessive amounts of ether groups, since otherwise thepolymers formed swell in water. The preferred polyether diols arepoly(oxypropylene) glycols in the molecular mass range M_(n) of 400 to3000.

Polyester diols are prepared by esterification of organic dicarboxylicacids or their anhydrides with organic diols or they are derived from ahydroxycarboxylic acid or a lactone. To prepare branched polyesterpolyols, polyols or polycarboxylic acids of a higher valency may be usedto a small extent. The dicarboxylic acids and diols may be linear orbranched aliphatic, cycloaliphatic or aromatic dicarboxylic acids ordiols.

The diols used for the preparation of the polyesters consist, forexample, of alkylene glycols, such as ethylene glycol, propylene glycol,butylene glycol, 1,4-butanediol, 1,6-hexanediol, neopentylglycol andother diols such as dimethylolcyclohexane. However, small amounts ofpolyols, such as trimethylolpropane, glycerol and pentaerythritol, maybe added. The acid component of the polyester consists primarily oflow-molecular dicarboxylic acids or their anhydrides having 2 to 30,preferably 4 to 18, carbon atoms in the molecule. Examples of suitableacids are o-phthalic acid, isophthalic acid, terephthalic acid,tetrahydrophthalic acid, cyclohexanedicarboxylic acid, succinic acid,adipic acid, azelaic acid, sebacic acid, maleic acid, fumaric acid,glutaric acid, hexachloroheptanedicarboxylic acid, tetrachlorophthalicacid and/or dimerized fatty acids. Instead of these acids it is alsopossible to use their anhydrides, provided they exist. In the formationof polyester polyols small amounts of carboxylic acids having 3 or morecarboxyl groups, for example trimellitic anhydride or the adduct ofmaleic anhydride with unsaturated fatty acids, may also be present.

It is also possible to use polyester diols which are obtained byreacting a lactone with a diol. They are distinguished by the presenceof terminal hydroxyl groups and recurring polyester moieties of theformula (--CO--(CHR²)_(n) --CH₂ --O). In this formula n is preferably 4to 6 and the substituent R² is hydrogen or an alkyl, cycloalkyl oralkoxy radical.

No substituent contains more than 12 carbon atoms. The total number ofcarbon atoms in the substituent does not exceed 12 per lactone ring.Corresponding examples are hydroxycaproic acid, hydroxybutyric acid,hydroxydecanoic acid and/or hydroxystearic acid.

The unsubstituted εΣ-caprolactone in which n has the value of 4 and allR substituents are hydrogen, is preferred for the preparation of thepolyester diols. The reaction with lactone can be initiated bylow-molecular polyols, such as ethylene glycol, 1,3-propanediol,1,4-butanediol and dimethylolcyclohexane. However, it is also possibleto react other reaction components, such as ethylenediamine,alkyldialkanolamines or even urea, with caprolactone.

Aliphatic and/or cycloaliphatic and/or aromatic polyisocyanates may beused as the component (ii). Examples of aromatic polyisocyanates arephenylene diisocyanate, tolylene diisocyanate, xylylene diisocyanate,biphenylene diisocyanate, naphthylene diisocyanate and diphenylmethanediisocyanate.

Because of their good resistance to ultraviolet light, (cyclo)aliphaticpolyisocyanates furnish products with a low tendency to yellowing.Examples of these are isophorone diisocyanate, cyclopentylenediisocyanate and hydrogenation products of aromatic diisocyanates suchas cyclohexylene diisocyanate, methylcyclohexylene diisocyanate anddicyclohexylmethane diisocyanate. Aliphatic diisocyanates are compoundsof the formula

    OCN--(CR.sup.3.sub.2).sub.r --NCO

in which r is an integer from 2 to 20, especially 6 to 8, and thesubstituents R³ which may be identical or different are hydrogen or alower alkyl radical having 1 to 8 carbon atoms, preferably 1 to 2 carbonatoms. Examples of these are trimethylene diisocyanate, tetramethylenediisocyanate, pentamethylene diisocyanate, hexamethylene diisocyanate,propylene diisocyanate, ethylethylene diisocyanate, dimethylethylenediisocyanate, methyltrimethylene diisocyanate and trimethylhexanediisocyanate. Particularly preferred diisocyanates are isophoronediisocyanate and dicyclohexylmethane diisocyanate.

The composition of the component (ii) in respect of the functionality ofthe polyisocyanates must be such that no crosslinked polyurethane resinis obtained. In addition to diisocyanates, the component (ii) may alsocontain a proportion of polyisocyanates having functionalities greaterthan two, for example triisocyanates.

Products which are formed by trimerization or oligomerization ofdiisocyanates or by reaction of diisocyanates with polyfunctionalcompounds containing OH or NH groups have been found to be suitabletriisocyanates. This group of compounds includes, for example, thebiuret of hexamethylene diisocyanate and water, the isocyanurate ofhexamethylene diisocyanate or the adduct of isophorone diisocyanate andtrimethylolpropane. The average functionality may be reduced, ifappropriate, by the addition of monoisocyanates. Examples of suchchain-terminating monoisocyanates are phenyl isocyanate, cyclohexylisocyanate and stearyl isocyanate.

In order to ensure the water-thinnability of the polyurethane resinsused it is necessary to incorporate in the polyurethane molecules groupscapable of forming anions. Groups capable of forming anions ensure,after being neutralized, that the polyurethane resin can be dispersed inwater to form a stable dispersion. The polyurethane resin should have anacid value of 10 to 60, preferably 20 to 35. The amount of groupscapable of forming anions to be introduced in the polyurethane moleculescan be calculated from the acid value.

The introduction of groups capable of forming anions in the polyurethanemolecule is carried out by incorporating compounds (iii).

Compounds containing in the molecule two groups reactive towardsisocyanate groups are preferably used as the component (iii). Suitablegroups reactive toward isocyanate groups are in particular hydroxylgroups as well as primary and/or secondary amino groups. Groups suitablefor forming anions are carboxyl, sulfonic acid and/or phosphonic acidgroups, carboxyl groups being preferred. Alkanoic acids having twosubstituents on the α carbon atoms may be used, for example, as thecomponent (iii). The substituent may be a hydroxyl group, an alkyl groupor, preferably, an alkylol group. These alkanoic acids have at leastone, generally 1 to 3, carboxyl groups in the molecule. They have two upto about 25, preferably 3 to 10 carbon atoms. Examples of the component(iii) are dihydroxypropionic acid, dihydroxysuccinic acid anddihydroxybenzoic acid. A group of alkanoic acids which is particularlypreferred comprises the α,α-dimethylolalkanoic acids of the generalformula R⁴ --C(CH₂ OH)₂ COOH, in which R⁴ is a hydrogen atom or an alkylgroup having up to about 20 carbon atoms. Examples of such compounds are2,2-dimethylolacetic acid, 2,2-dimethylolpropionic acid,2,2-dimethylolbutyric acid and 2,2-dimethylolpentanoic acid.2,2-Dimethylolpropionic acid is the preferred dihydroxyalkanoic acid.Examples of compounds containing amino groups are α,δ-diaminovalericacid, 3,4-diaminobenzoic acid, 2,4-diaminotoluenesulfonic acid and2,4-diaminodiphenyl ether sulfonic acid.

The polyurethane resins used according to the invention may be prepared,if appropriate, by a concomitant use of organic compounds containinghydroxyl and/or amino groups and having a molecular weight of 40 to 600,or a mixture of such compounds (component (iv)). The use of thecomponent (iv) leads to an increase in molecular weight of thepolyurethane resins. For example, polyols having up to 20 carbon atomsper molecule, such as ethylene glycol, diethylene glycol, triethyleneglycol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, 1,2-butyleneglycol, 1,6-hexanediol, trimethylolpropane, castor oil or hydrogenatedcastor oil, di(trimethylolpropane) ether, pentaerythritol,1,2-cyclohexanediol, 1,4-cyclohexanedimethanol, bisphenol A, bisphenolF, neopentyl glycol, neopentyl glycol hydroxypivalate, hydroxyethylatedor hydroxypropylated bisphenol A, hydrogenated bisphenol A and mixturesthereof may be used as the component (iv).

In general, the polyols are used in amounts of up to 30 percent byweight, preferably 2 to 20 percent by weight, based on the total amountof the components (i) and (iv). Diamines and/or polyamines with primaryand/or secondary amino groups may also be used as the component (iv).The polyamines are essentially alkylenepolyamines having 1 to 40 carbonatoms, preferably about 2 to 15 carbon atoms. They may containsubstituents which are free from any hydrogen atoms capable of reactingwith isocyanate groups. Examples are polyamines with a linear orbranched aliphatic, cycloaliphatic or aromatic structure and at leasttwo primary amino groups. Examples of suitable diamines are hydrazine,ethylenediamine, propylenediamine, 1,4-butylenediamine, piperazine,1,4-cyclohexyldimethylamine, 1,6-hexamethylenediamine,trimethylhexamethylenediamine, menthanediamine, isophoronediamine,4,4'-diaminodicyclohexylmethane and aminoethylethanolamine. Preferreddiamines are hydrazine, alkyl- or cycloalkyldiamines, such aspropylenediamine and 1-amino-3-aminomethyl-2,5,5-trimethylcyclohexane.Polyamines containing more than two amino groups in the molecule may bealso used as the component (iv). However, in such cases care must betaken, for example by a concomitant use of monoamines, that nocrosslinked polyurethane resins are obtained. Polyamines of this typewhich can be used are diethylenetriamine, triethylenetetramine,dipropylenetriamine and dibutylenetriamine. Ethylhexylamine is anexample of a monoamine.

Poly(oxyalkylene) groups may be introduced in the polyurethane moleculesas non-ionic stabilizing groups with the aid of the component (v).Alkoxypoly(oxyalkylene) alcohols of the general formula R'O--(--CH₂--CHR"--O--)_(n) H, in which R' is an alkyl radical having 1 to 6 carbonatoms, R" is a hydrogen atom or an alkyl radical having 1 to 6 carbonatoms and n is a number between 20 and 75, may be used, for example, asthe component (v).

The preparation of the polyurethane resins which can be used incombination with the polyacrylate resin essential to the inventionbelongs to the prior art and is described in detail, for example, inU.S. Pat. No. 4,719,132, DE-A-3,628,124, EP-A-89,497, EP-A-256,450 andWO 87/03829.

The water-thinnable polyester resins to be used in combination with thepolyacrylate resins essential to the invention may be prepared frompolycarboxylic acids and polyols by generally well known methods. Any ofthe starting materials listed in the description of the component (i)may be used for the preparation of the polyester resins.

The polyester resins which are preferably used as the water-thinnablepolyester resins can be obtained by reacting

(α) polyols or a mixture of polyols and

(β) polycarboxylic acids or polycarboxylic anhydrides, or a mixture ofpolycarboxylic acids and/or polycarboxylic anhydrides to form apolyester resin having a number average molecular weight of 600 to 5000,preferably 800 to 2500, an acid value of 20 to 70, preferably 25 to 55,and a hydroxyl value of 30 to 200, preferably 45 to 100, in whichreaction

the components (α) and (β) are used in a molar ratio of 1.15-2.00:1,preferably 1.2-1.5:1,

the component (α) consists of 30 to 100 mol % of aliphatic diolscontaining at least one α carbon atom which is secondary, tertiary or amember of a carbon-containing ring system, and

the component (β) consists of 50 to 100 mol % of aromatic and/orcycloaliphatic polycarboxylic acids and of 15 to 40 mol % oftricarboxylic and/or tetracarboxylic acids, the tricarboxylic and/ortetracarboxylic acids being used in such a way that they areincorporated in the polyester resin molecules via at least two carboxylgroups.

The component (α) consists of

(α1) 30 to 100, preferably 50 to 100 mol % of aliphatic diols whichcontain at least one α carbon atom which is secondary, tertiary or amember of a carbon-containing ring system,

(α2) 0 to 20, preferably 0 to 10 mol % of aliphatic triols and

(α3) 0 to 40, preferably 0 to 20 mol % of diols containing ether groups.

In principle any aliphatic diol having 4 to 40, preferably 5 to 12carbon atoms in the molecule, in which at least one α carbon atom is asecondary or tertiary carbon atom or a member of a carbon-containingring system, may be used as the component (α1). Mixtures of such diolsmay also be used. Compounds which may be used as the component (α1) are,for example, those containing at least one molecular fragment of thegeneral formula --C(R¹ R²)--CH₂ OH, R¹ and R² being aliphatic,cycloaliphatic or aromatic hydrocarbon radicals having 1 to 20,preferably 1 to 6, carbon atoms. Examples of such compounds areneopentyl glycol, 2-methyl-2-propyl-l,3-propanediol,2-ethyl-2-butyl-1,3-propanediol, 2-ethyl-2-phenyl-1,3-propanediol,2,2,4-trimethyl-1,5-pentanediol, 2,2,5-trimethyl-1,6-hexanediol andneopentyl glycol hydroxypivalate. An example of a compound in which atleast one α carbon atom is a member of a carbon-containg ring system, isdimethylolcyclohexane. Neopentyl glycol, neopentyl glycolhydroxypivalate, dimethylolcyclohexane and2-ethyl-2-butyl-1,3-propanediol are preferably used as the component(α1).

Glycerol, trimethylolpropane and trimethylolethane, for example, may beused as the component (α2).

Diols which are used as the component (α3) in particlular those whichcontain 1 to 10 ether oxygen atoms in the molecule or mixtures of suchcompounds. The following are examples of the component (α3): diethyleneglycol, dipropylene glycol, triethylene glycol, tripropylene glycol,tetraethylene glycol, tetrapropylene glycol and poly(ethylene oxide),poly(propylene oxide) and poly(ethylene oxide) (propylene oxide) havingnumber average molecular weights of 400 to 600.

The component (β) consists of

(β1) 50 to 100, preferably 50 to 80 mol % of aromatic and/orcycloaliphatic polycarboxylic acids or mixtures of such polycarboxylicacids, and

(β2) 0 to 50, preferably 20 to 50 mol % of aliphatic polycarboxylicacids or mixtures of aliphatic carboxylic acids,

the proportion of tricarboxylic or tetracarboxylic acids being 15 to 40mol %. Reactive carboxylic acid derivatives, such as carboxylicanhydrides, may of course also be used instead of the carboxylic acids.

In principle any cycloaliphatic or aromatic polycarboxylic acid having 5to 30, preferably 6 to 18 carbon atoms in the molecule or an anhydrideof this polycarboxylic acid or a mixture of these polycarboxylic acidsor their anhydrides may be used as the component (β1). Examples ofpolycarboxylic acids which may be used are isophthalic acid,terephthalic acid, orthophthalic acid, tetrahydrophthalic acid,hexahydrophthalic acid, 1,4-cyclohexanedicarboxylic acid,dicyclopentadienedicarboxylic acid, trimesic acid(benzene-1,3,5-tricarboxylic acid), trimellitic acid, pyromellitic acidand endomethylenetetrahydrophthalic acid as well as their anhydrides.Isophthalic acid, terephthalic acid, orthophthalic acid,tetrahydrophthalic acid, hexahydrophthalic acid, trimellitic acid,pyromellitic acid, their anhydrides or mixtures of these polycarboxylicacids or of their anhydrides are preferably used as the component (β1).

In principle any linear or branched aliphatic polycarboxylic acid having2 to 40 carbon atoms in the molecule or an anhydride of thesepolycarboxylic acids or a mixture of these polycarboxylic acids or theiranhydrides may be used as the component (β2). Examples of aliphaticpolycarboxylic acids which may be used are oxalic acid, malonic acid,succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid,azelaic acid, sebacic acid, maleic acid, fumaric acid, polymerized fattyacids and citric acid as well as their anhydrides. Adipic acid, azelaicacid, sebacic acid, succinic acid, their anhydrides or mixtures of thesepolycarboxylic acids or their anhydrides are preferably used as thecomponent (β2). Polymerized fatty acids, especially dimerized fattyacids, are quite particularly preferred as the component (β2). Whenpolymerized fatty acids are used as the component (β2), basecoats areobtained having a particularly good shelf life.

Polymeric fatty acids are generally prepared by polymerizing fattyacids, for example linolenic, linoleic or oleic acid individually, inadmixture with each other or in admixture with saturated fatty acids. Amixture is obtained which contains mainly dimeric, but also monomericand trimeric molecules as well as by-products, depending on the conductof the reaction. The products are normally purified by distillation.Commercial polymeric fatty acids generally contain at least 80% byweight of dimeric fatty acids, up to 20% by weight of trimeric fattyacids and not more than 1% by weight of monomeric fatty acids. It ispreferred to use as the component (β2) polymeric fatty acids whichconsists of at least 98% by weight of dimeric fatty acids and of notmore than 2% by weight of trimeric fatty acids and not more than tracesof monomeric fatty acids.

Polymeric fatty acids contain both cyclic and linear aliphatic molecularfragments. However, in the context of the present inventions they areconsidered not as cycloaliphatic but as linear aliphatic polycarboxylicacids and are thus included under component (β2).

Trimellitic acid or pyromellitic acid, mixtures thereof or anhydridesthereof, are preferably used as the tricarboxylic or tetracarboxylicacids.

The preparation of the water-thinnable polyester resins is carried outaccording to generally well known methods of polyester chemistry byreacting the component (a) with the component (β). The reactiontemperature should expediently be about 140 to 240, preferably 180° to220° C. It may be expedient in some cases to catalyze the esterificationreaction. Examples of catalysts which can be used are tetrabutyltitanate, zinc octoate, tin octoate, dibutyltin oxide, organic salts ofdibutyltin oxide etc. Care must be taken during the esterification thatthe tricarboxylic or tetracarboxylic acids are incorporated into thepolyester resin molecules in such a way that on statistical average atleast two carboxyl groups are esterified.

Ammonia and/or amines (in particular alkylamines), aminoalcohols andcyclic amines, such as diethylamine and triethylamine,dimethylaminoethanolamine, diisopropanolamine, morpholine,N-alkylmorpholine etc. may be used for the neutralization of thepolyester resins. Highly volatile amines are preferred for theneutralization.

The amino resins which can be used in combination with the polyacrylateresins essential to the invention are available from many companies ascommercial products (for example Cymel® from American Cyanamid Company,Resimene® from Monsanto Company and Luwipal® from BASF AG). Usually theyare at least partially etherified condensation products of compoundscontaining amino groups, in particular melamine or benzoguanamine andaldehydes, in particular formaldehyde. The water-thinnability of theamino resins generally depends on the degree of condensation and on theetherification component. The lower the degree of condensation and theshorter the chains of the alkyl groups in the etherification component,the better is the water-thinnability of the amino resins. Thewater-thinnability of amino resins may also be enhanced by theintroduction of carboxyl groups (for example etherification usinghydroxycarboxylic acids). Furthermore, water-thinnability of aminoresins may be enhanced by the addition of water-thinnable solvents, forexample glycol ethers.

In addition to the binders described above, the basecoats according tothe invention may contain other water-thinnable synthetic resins whichare used for grinding of the pigments and/or as rheology-controllingadditives. Examples of such synthetic resins are polyethers, for examplepolypropylene glycol having a number average molecular weight of 400 to900, water-soluble cellulose ethers such as hydroxyethyl cellulose,methyl cellulose or carboxymethyl cellulose as well as syntheticpolymers containing ionic and/or associatively acting groups, such aspolyvinyl alcohol, poly(meth)acrylamide, poly(meth)acrylic acid,polyvinylpyrrolidone, styrene-maleic anhydride or ethylene-maleicanhydride copolymers and their derivatives or hydrophobicity modifiedethoxylated urethanes, or polyacrylates containing carboxyl groups.

The basecoats according to the invention may also contain crosslinkedpolymicroparticles, such as those disclosed, for example, inEP-A-38,127.

The basecoats according to the invention may also contain inorganicrheology-controlling agents, for example phyllosilicates.

The basecoats according to the invention may contain as pigmentschromophoric inorganic pigments, for example titanium dioxide, ironoxide, carbon black etc. and/or chromophoric organic pigments and/orconventional metallic pigments (for example commercial aluminum bronzes,stainless steel bronzes . . . ) and/or non-metallic special-effectpigments (for example pearlescent pigments or interference pigments).The basecoats according to the invention preferably contain metallicpigments and/or special-effect pigments. The degree of pigmentation iswithin customary levels.

At spraying viscosity, the basecoats according to the inventiongenerally have a solids content of about 15 to 50% by weight. The solidscontent varies according to the intended application of the basecoats.That of metallic paints is preferably, for example, 17 to 25% by weight.For solid-color paints it is higher, for example about 30 to 45% byweight.

The basecoats according to the invention may additionally containconventional organic solvents. Their proportion is kept as low aspossible. It is, for example, below 15% by weight.

The pH of the basecoats according to the invention is generally adjustedto between 6.5 and 9.0. The pH can be adjusted with conventional amines,for example triethylamine, dimethylaminoethanol and N-methylmorpholine.

The basecoats according to the invention may be used both in productionline finishing and in refinishing. They are preferably used inproduction line finishing.

Organic solvent-borne paints, water-thinnable paints and powder paintsmay be used as the transparent topcoats. The paints may be used asunpigmented clearcoats or as transparently pigmented paints.

Using the basecoats according to the invention, it is possible toproduce a high-quality finish even without recoating with a transparenttopcoat. One-coat finishes are obtained in this manner which aredistinguished by a particularly high gloss.

The paints according to the invention may be applied to any substrate,for example metal, wood, plastic or paper.

In the examples below the invention is elucidated in greater detail.

All parts and percentages are by weight, unless expressly statedotherwise.

A. Preparation of polyacrylate resins to be used according to theinvention

A 1

32 parts by weight of butyl glycol are introduced into a steel reactionvessel fitted with monomer feed, initiator feed, thermometer, oilheating and reflux condenser, and the charge is heated to 110° C. Asolution of 6.0 parts by weight of t-butyl perethylhexanoate in 6.0parts by weight of butyl glycol is then added at such a rate that theaddition is concluded after 5 h 30 min.

At the same time as the addition of the t-butyl perethylhexanoatesolution commences, the addition commences of a mixture consisting of(a1): 21.6 parts by weight of n-butyl methacrylate, 20.0 parts by weightof methyl methacrylate and 20.0 parts by weight of lauryl methacrylate;(a2): 20.4 parts by weight of hydroxypropyl acrylate and (a3): 15.0parts by weight of styrene. The mixture of (a1), (a2) and (a3) is addedat such a rate that the addition is concluded in 5 h.

When the t-butyl perethylhexanoate solution has been completely added,the polymerization temperature is kept at 110° C. for a further 1 h.

Subsequently a solution of 1.56 parts by weight of t-butylperethylhexanoate in 3.5 parts by weight of butyl glycol is added atsuch a rate that the addition is concluded after 1 h 30 min. At the sametime as the addition of the t-butyl perethylhexanoate solutioncommences, the addition commences of a mixture consisting of (b1): 5.6parts by weight of acrylic acid and (b2): 3.1 parts by weight of butylmethacrylate, 1.96 parts by weight of methyl methacrylate and 3.93 partsby weight of lauryl methacrylate and 1.96 parts by weight of styrene.

The mixture of (b1) and (b2) is added at such a rate that the additionis concluded after 1 h.

The temperature is kept at 110° C. for a further 1 h 30 min. The resinsolution obtained in this way is concentrated by vacuum distillation to80% by weight (solids content) and neutralized with dimethylethanolamineat about 80° C. to an 80% degree of neutralization over about 30 min.The resin solution is cooled to 60° C. and the heating is arrested.

Water is then slowly added until the solids content of the dispersion isabout 40% by weight.

The resultant dispersion has the following characteristics: acid value35.8 mg of KOH/g, hydroxyl value: 80, number average molecular weight:4990, particle size 221 nm¹

A2

35.8 parts by weight of butyl glycol are introduced into a steelreaction vessel fitted with monomer feed, initiator feed, thermometer,oil heating and reflux condenser and the charge is heated to 110° C. Asolution of 3.5 parts by weight of t-butyl perethyl-hexanoate in 7.0parts by weight of butyl glycol is then added at such a rate that theaddition is concluded after 5 h 30 min.

At the same time as the addition of the t-butyl perethylhexanoatesolution commences, the addition commences of a mixture consisting of(a1): 22.0 parts by weight of n-butyl acrylate, 20.0 parts by weight oft-butyl acrylate and 15.0 parts by weight of methyl methacrylate; (a2):23.0 parts by weight of hydroxypropyl acrylate and (a3): 15.0 parts byweight of styrene. The mixture of (a1), (a2) and (a3) is added at such arate that the addition is concluded after 4 h 50 min.

When the mixture of (a1), (a2) and (a3) has been completely added, (b1):5.0 parts by weight of acrylic acid are added at such a rate that theaddition is concluded after 20 min.

The temperature is kept at 110° C. for a further 1 h 50 min. The resinsolution obtained in this way is cooled to 80° C. and neutralized withdimethylethanolamine at about 80° C. to an 82.5% degree ofneutralization over about 30 min. The resin solution is cooled to 60° C.and the heating is arrested.

Water is then slowly added until the solids content of the dispersion isabout 40% by weight.

The resultant dispersion has the following characteristics: acid value36.5 mg of KOH/g, hydroxyl value: 100, particle size: 267, solidscontent (in % by weight, 1 h, 130° C.): 40.0.

B. Preparation of a polyacrylate resin which is not according to theinvention

35.8 parts by weight of butyl glycol are introduced into a steelreaction vessel fitted with monomer feed, initiator feed, thermometer,oil heating and reflux condenser and the charge is heated to 110° C. Asolution of 3.5 parts by weight of t-butyl perethylhexanoate in 7.0parts by weight of butyl glycol is added at such a rate that theaddition is concluded after 5 h 30 min. At the same time as the additionof the t-butyl perethylhexanoate solution commences, the additioncommences of a mixture of 22.0 parts by weight of n-butyl acrylate, 20.0parts by weight of t-butyl acrylate, 15.0 parts by weight of methylmethacrylate, 15.0 parts by weight of styrene, 23.0 parts by weight ofhydroxypropyl acrylate and 5.0 parts by weight of acrylic acid. Themonomer mixture is added at such a rate that the addition is concludedafter 5 h 10 min. The further procedure follows that of A 2.

The resultant dispersion has the following characteristics: acid value:39.0 mg of KOH/g, hydroxyl value: 100, solids content (in % by weight, 1h, 130° C.): 40.0.

C. Preparation of an aqueous polyurethane resin dispersion

569 parts by weight of a condensation product (number average molecularweight 1460) obtained from 1 mol of a polymeric fatty acid (dimercontent at least 98% by weight, trimer content not more than 2% byweight, monomer content not more than traces), 1 mol of isophthalic acidand 2.626 mol of hexanediol, 46 parts by weight of dimethylolpropionicacid, 7 parts by weight of neopentyl glycol, 420 parts by weight ofmethyl ethyl ketone and 213 parts by weight of isophorone diisocyanateare heated to 80° C. in an atmosphere of nitrogen in atemperature-controllable reaction vessel fitted with a stirrer and waterseparator. The reaction is allowed to proceed to an NCO content of 1.0%by weight, based on the total composition. 24 parts by weight oftrimethylolpropane are added and the reaction mixture is stirred at 80°C. until no isocyanate groups are detectable.

25.8 parts by weight of dimethylethanolamine and subsequently 2552 partsby weight of deionized water are slowly added with stirring. The methylethyl ketone is removed by vacuum distillation.

A finely divided dispersion is obtained having a pH of 7.8, anon-volatile content of 27% by weight and an acid value of 25 mg ofKOH/g.

D. Preparation of an aqueous polyester resin dispersion

729 parts by weight of neopentyl glycol, 827 parts by weight ofhexanediol, 462 parts by weight of hexahydrophthalic anhydride and 1710parts by weight of a polymeric fatty acid (dimer content at least 98% byweight, trimer content not more than 2% by weight, monomer content notmore than traces) are weighed into a reaction vessel fitted with astirrer, a thermometer and a packed column, and the mixture is melted.The rate of heating, with stirring, is such that the temperature at thehead of the column does not exceed 100° C. Esterification is allowed toproceed at 220° C. max. until an acid value of 8.5 is reached. Thereaction mixture is allowed to cool to 180° C., 768 parts by weight oftrimellitic anhydride are added and esterification is allowed to proceedfurther until an acid value of 30 is reached. The mixture is then cooledto 120° C. and brought into solution with 1410 parts by weight ofbutanol. The mixture is allowed to cool to 90° C. and 16.2 parts byweight of dimethylethanolamine and subsequently 1248 parts by weight ofdeionized water are slowly added with stirring. A finely divideddispersion is obtained having a pH of 7.8, a non-volatile content of 60%by weight and an acid value of 30 mg of KOH/g.

E. Production of an aqueous metallic basecoat (Comparison Example)

33.5 parts by weight of a thickener (3% paste of a sodium-magnesiumphyllosilicate* in water) are treated with a solution consisting of 4.3parts by weight of

F. Production of an aqueous metallic basecoat according to the invention

33.5 parts by weight of a thickener (3% paste of a sodium-magnesiumphyllosilicate* in water) are treated with a solution of 4.3 parts byweight of butyl glycol and 6.0 parts by weight of a 90% solution of acommercial water-thinnable melamine-formaldehyde resin in isobutanol(Cymel 327®) with stirring. 33.3 parts by weight of the polyurethaneresin dispersion from C, 0.4 part by weight of dimethylethanolaminesolution (10% in water) and 4.8 parts by weight of the polyacrylateresin dispersion from A 1 are added successively to this mixture withstirring. An aluminum pigment suspension is prepared separately asfollows: 4.4 parts by weight of a commercial chromatized aluminum paste(65% in petroleum spirit/solvent naphtha/butyl glycol, average particlediameter: 15 μm) are homogenized by the addition of 4 parts by weight ofbutyl glycol. 3.2 part by weight of the water-soluble polyester resinfrom D and 1.0 part by weight of polypropylene glycol (number averagemolecular weight: 900) are then added to this suspension. This aluminumpigment suspension is added to the mixture described above withstirring. 3.8 parts by weight of deionized water are added subsequentlyand the pH of the mixture is adjusted to 7.65-7.85 withdimethylethanolamine solution (10% in water).

G. Application and testing of the aqueous metallic basecoats

The aqueous metallic basecoats produced in E and F are adjusted to anapplication solids content of 24.2% by weight with distilled water andare applied to a phosphated steel panel coated with a commercialelectrodeposition primer [l+f glossay] and a commercial body filler insuch a way that a dry film thickness of 12-15 μm is obtained. Theapplied basecoats are dried for 10 minutes at room temperature and for10 minutes at 80° C. in a circulating air oven. A commercialtwo-component clearcoat based on polyacrylate/polyisocyanate is thenapplied, the panel is then flashed off for a brief period and baked at140° C. in a circulating air oven for 20 min.

    ______________________________________                                                         Basecoat Basecoat                                            Test results:    from E   from F                                              ______________________________________                                        L 25° (1) 102.7    103.2                                               L 70° (1) 40.4     40.9                                                Gloss (2)        83       84                                                  DOI (3)          92       92                                                  Crosshatch (4)   0        0                                                   Gloss after humidity                                                                           81       81                                                  test (5)                                                                      Crosshatch after 1        1                                                   humidity test                                                                 DOI after        91       91                                                  humidity test                                                                 ______________________________________                                         (1) Color content L* according to DIN 6174, determined using the Zeiss        goniospectrophotometer.                                                       (2) Degree of gloss according to DIN 67530, angle of 20                       (3) Distinctness of reflected image: with the surface to be assessed          illuminated at an angle of 30°, the direct reflection is measured      at an angle of incidence of -30 ° and in the immediate proximity o     the angle of incidence at -30° C. ± 0.3°. The DOI value      thus determined corresponds with the visually perceived sharpness of the      reflected image of an object on this surface. The DOI value is also           referred to as image sharpness value. Rating: 100 best value: 0 worst         value.                                                                        (4) Test according to DIN 53151 including the Tesa peeloff test.              (5) Humidity test at 40° C. for 240 h (according to DIN 50017)    

Test of viscosity stability under shearing stress

To test viscosity stability under shearing stress, clearcoat media(clearcoat medium=basecoat without pigments) of the basecoats from E andF are vigorously stirred for 1 h using a paddle stirrer, are thensheared in a viscometer at a shear rate of 0 to 50 s⁻¹ and theviscosities are measured. The clearcoat medium from the basecoat from Eexhibited a viscosity drop of about 40%. On the other hand, theclearcoat medium from the basecoat from F exhibited a viscosity drop ofless than 10%. This low viscosity drop has no negative effects onapplication characteristics and sedimentation behaviour.

H. Production of a pigment paste for aqueous solid basecoats

10 parts by weight of a thickener (3% paste of a sodium-magnesiumphyllosilicate in water) is treated with 4.5 parts by weight ofdimethylethanolamine solution (10% in water), 2 parts by weight ofpolypropylene glycol (molecular weight 900), 46.70 parts by weight ofthe polyester resin from D and 23.35 parts by weight of deionized waterwith stirring. The following pigments are then added individually withstirring: 2.65 parts by weight of Irgazin Red DPP BO®¹, 4.38 parts byweight of Cromophthal Red A2B®¹, 5.29 parts by weight of Novoperm OrangeH2 70®² and 1.13 parts by weight of Sicotan Yellow 2 1912®³. The mixtureis finally homogenized for 20 minutes using a conventional laboratorydissolver. The mixture is then ground in a conventional laboratory beadmill to a Hegmann (ISO 152) particle fineness of ≦5 μm.

The pH of the pigment paste should be between 8 and 8.5, failing whichthe pH is adjusted to this value using a dimethylethanolamine solution(10% in water).

I. Production of aqueous solid basecoats

Two solid basecoats are produced using the pigment paste and thecomponents listed below, the solid basecoat 2 being used as a comparisonexample. The weight ratio of pigment to solid resin and the weight ratioof OH-containing solid resin to melamine-formaldehyde resin are the samein both basecoats.

    ______________________________________                                                         Solid   Solid                                                                 basecoat 1                                                                            basecoat 2                                           ______________________________________                                        Pigment paste      41.33     33.57                                            Polyacrylate resin from A 2                                                                      25.77     --                                               Polyacrylate resin from B                                                                        --        20.90                                            Luwipal LR 8789    5.88      4.77                                             Resimene 4518      0.92      0.75                                             Deionized water    26.10     40.01                                            ______________________________________                                    

The above constituents are mixed successively with stirring. The figuresquoted are parts by weight. The viscosities of the basecoats obtained inthis way are adjusted to 28 s (DIN 4). The solid basecoat 1 has at thisviscosity a solids content of 34% by weight (1 h, 130° C.) and the solidbasecoat 2 a solids content of 28.3% by weight (1 h, 130° C.).

J. Application and testing of the solid basecoats

The aqueous solid basecoats produced in I. are stored for 48 hours andthen applied to phosphated steel panels coated with a commercialelectrodeposition primer [L+F glossary] and a commercial body filler bythe following procedure: compressed air jet spray application at arelative atmospheric humidity of 60% and a temperature of 23° C.,two-coat application with a flash-off time of 2 minutes after the firstapplication, a flash-off time of 1 minute after the second applicationand 10 minutes' drying in a circulating air oven at 80° C. Thecommercial clearcoat based on polyacrylate/melamineformaldehyde resin isthen applied, followed by a brief flash-off period and baking for 30minutes at 130° C.

A test of the resultant two-coat finish obtained in this way gave thefollowing results:

    ______________________________________                                                     Two-coat finish                                                                          Two-coat finish                                                    using solid                                                                              using solid                                                        basecoat 1 basecoat 2                                            ______________________________________                                        Stone-chip     2            2-3                                               resistance test                                                               according to VDA.sup.1                                                        Crosshatch test                                                                              0            0                                                 Gloss at 20°                                                                          87           87                                                Blister limit.sup.2                                                                          48 μm     28 μm                                          Run limit.sup.3                                                                              48 μm     24 μm                                          ______________________________________                                         .sup.1 VDA: Verband der deutschen Automobilindustrie; blasting medium:        1000 g of bevelled shot; particle size: 4-5 mm; pressure: 1.5 bar; VDA        stonechip resistance tester 508                                               .sup.2 Application by the described method; basecoat film thicknesses at      which blistering occurs                                                       .sup.3 Application by the described method; basecoat film thicknesses at      which runs appear in the basecoat film when applied to vertical perforate     panels and subsequently predried in a vertical position.                 

We claim:
 1. A process for the production of a multicoat finish,including the steps of:(1) applying an aqueous, pigmented basecoat to asubstrate surface as basecoat, (2) forming a polymer film from thecoating applied in stage (1), (3) applying a transparent topcoat to thebasecoat, and subsequently (4) baking the basecoat together with thetopcoat; wherein the basecoat contains a water-thinnable, uncrosslinkedpolyacrylate resin prepared by a solution polymerization processconsisting essentially of the steps of: (I) polymerizing(a1) 40 to 90%by weight of a (meth)acrylic acid ester essentially free from carboxylgroups selected from the group consisting of alkyl acrylates, alkylmethacrylates, cycloaliphatic acrylic acid esters, cycloaliphaticmethacrylic acid esters, and mixtures thereof, (a2) 0 to 45% by weightof an ethylenically unsaturated monomer which contains at least onehydroxyl group per molecule and is essentially free from carboxylgroups, or a mixture of such monomers, and (a3) 0 to 40% by weight of anethylenically unsaturated monomer essentially free from carboxyl groups,which is different from (a 1) and (a2), in an organic solvent or mixtureof solvents in the presence of at least one polymerization initiator,and (II) adding, after at least 80% by weight of the monomers in stage(I) have reacted,(b1) 2.5 to 15% by weight of an ethylenicallyunsaturated monomer containing at least one carboxyl group per molecule,or a mixture of such monomers, and (b2) 0 to 60% by weight of anethylenically unsaturated monomer essentially free from carboxyl groups,or a mixture of such monomers, and continuing the polymerization toobtain the polyacrylate resin, and (III) neutralizing, at leastpartially, the polyacrylate resin and dispersing it in water;wherein thesum of (a1), (a2), (a3), (b1), and (b2) is 100% and further wherein theuncrosslinked polyacrylate resin has a hydroxyl value of 0 to 200, anacid value of 20 to 100, a glass transition temperature (T_(G)) of -40°to +60° C., and a molecular weight of between 2,500 and 20,000.
 2. Theprocess as claimed in claim 1 wherein the basecoat further contains ametallic pigment or a mixture of metallic pigments.
 3. The process asclaimed in claim 1, wherein polymerization is carried out in stages (I)and (II) at a temperature of 80° to 160° C. in the presence of at leastone free radical-forming initiator, the duration of stage (I) is 2 to 8hours, and the stage (II) addition is carried out over a period of 10 to90 minutes.
 4. The process as claimed in claim 1, wherein the stage (II)addition is begun after at least 90% by weight of the monomers added instage (I) have reacted.
 5. The process as claimed in claim 1, whereinthe basecoat further comprises a resin selected from the groupconsisting of a water-thinnable polyurethane resin, a water-thinnablepolyester resin, a water-thinnable amino resin, and mixtures thereof. 6.The process as claimed in claim 1, wherein the basecoat comprises atleast one non-metallic pigment and a mixture consisting of(A) 10 to 95%by weight of the water-thinnable polyacrylate resin, (B) 5 to 50% byweight of an amino resin, (C) 0 to 85% by weight of a water-thinnablepolyester resin, and (D) 0 to 85% by weight of a water-thinnablepolyurethane resin,the sum of components (A) to (D) being 100%.
 7. Theprocess as claimed in claim 1, wherein the basecoat comprises a metallicpigment and a mixture consisting of(A) 0.1 to 60% by weight of thewater-thinnable polyacrylate resin, (B) 0 to 50% by weight of an aminoresin, (C) 0 to 50% by weight of a water-thinnable polyester resin, and(D) 10 to 99.9% by weight of a water-thinnable polyurethane resin,thesum of components (A) to (D) being 100%.
 8. The process as claimed inclaim 5, wherein the basecoat comprises a water-thinnable polyesterresin obtained by reacting(α) a polyol or a mixture of polyols,consisting of 30 to 100 mol % of aliphatic diols containing at least oneα carbon that is secondary, tertiary, or in a ring system, and (β) amember selected from the group consisting of polycarboxylic acids,polycarboxylic anhydrides, and mixtures thereof, wherein component (β)consists of 50 to 100 mol % of aromatic or cycloaliphatic polycarboxylicacids or mixtures thereof, and 15 to 40 mol % of tricarboxylic ortetracarboxylic acids or mixtures thereof, with the caveat that at leasttwo carboxyl groups of the tricarboxylic and tetracarboxylic acids reacton statistical average, in a molar ratio of components (α) and (β) of1.15-2.00:1; and further wherein the polyester resin is at leastpartially neutralized and has a number average molecular weight of 600to 5000, an acid value of 20 to 70, and a hydroxyl value of 30 to 200.9. The process as claimed in claim 8, wherein the component (β) consistsof 50 to 80 mol % of aromatic or cycloaliphatic polycarboxylic acids ormixtures thereof, and 20 to 50 mol % of one or more polymeric fattyacids, wherein the polymeric fatty acids are not cycloaliphaticpolycarboxylic acids.
 10. An aqueous, pigmented paint containing awater-thinnable, uncrosslinked polyacrylate resin prepared by a solutionpolymerization process consisting essentially of(I) polymerizing(a1) 40to 90% by weight of a (meth)acrylic acid ester essentially free fromcarboxyl groups selected from the group consisting of alkyl acrylates,alkyl methacrylates, cycloaliphatic acrylic acid esters, cycloaliphaticmethacrylic acid esters, and mixtures thereof, (a2) 0 to 45% by weightof an ethylenically unsaturated monomer which contains at least onehydroxyl group per molecule and is essentially free from carboxylgroups, or a mixture of such monomers, and (a3) 0 to 40% by weight of anethylenically unsaturated monomer essentially free from carboxyl groups,which is different from (a1) and (a2), or a mixture of such monomers, inan organic solvent or mixture of solvents in the presence of at leastone polymerization initiator, and (II) adding, after at least 80% byweight of the monomers in stage (I) have reacted,(b 1) 2.5 to 15% byweight of an ethylenically unsaturated monomer containing at least onecarboxyl group per molecule, or a mixture of such monomers, and (b2) 0to 60% by weight of an ethylenically unsaturated monomer essentiallyfree from carboxyl groups, or a mixture of such monomers, and continuingthe polymerization to obtain the polyacrylate resin, and (III)neutralizing, at least partially, the polyacrylate resin and dispersingit in water;wherein the sum of (a1), (a2), (a3), (b1) and (b2) is 100%and further wherein the polyacrylate resin has a hydroxyl value of 0 to200, an acid value of 20 to 100, a glass transition temperature (T_(G))of -40° to +60° C., and a molecular weight of between 2,500 and 20,000.11. The paint as claimed in claim 10 wherein the paint further containsa metallic pigment or a mixture of metallic pigments.
 12. The paint asclaimed in claim 10, wherein polymerization is carried out in stages (I)and (II) at a temperature of 80° to 160° C. in the presence of at leastone free radical-forming initiator, the duration of stage (I) is 2 to 8hours, and the stage (II) addition is carried out over a period of 10 to90 minutes.
 13. The paint as claimed in claim 10, wherein the stage (II)addition is begun after at least 90% by weight of the monomers added instage (I) have reacted.
 14. The paint as claimed in claim 10, whereinthe paint further comprises a resin selected from the group consistingof a water-thinnable polyurethane resin, a water-thinnable polyesterresin, a water-thinnable amino resin, and mixtures thereof.
 15. Thepaint as claimed in claim 10, wherein the paint comprises at least onenon-metallic pigment and a mixture consisting of(A) 10 to 95% by weightof the water-thinnable polyacrylate resin, (B) 5 to 50% by weight of anamino resin, (C) 0 to 85% by weight of a water-thinnable polyesterresin, and (D) 0 to 85% by weight of a water-thinnable polyurethaneresin,the sum of components (A) to (D) being 100%.
 16. The paint asclaimed in claim 10, wherein the paint comprises a metallic pigment anda mixture consisting of(A) 0.1 to 60% by weight of the water-thinnablepolyacrylate resin, (B) 0 to 50% by weight of an amino resin, (C) 0 to50% by weight of a water-thinnable polyester resin, and (D) 10 to 99.9%by weight of a water-thinnable polyurethane resin,the sum of components(A) to (D) being 100%.
 17. The paint as claimed in claim 14, wherein thepaint comprises a water-thinnable polyester resin obtained byreacting(α) a polyol or a mixture of polyols, consisting of 30 to 100mol % of aliphatic diols containing at least one α carbon that issecondary, tertiary, or in a ring system, and (β) a member selected fromthe group consisting of polycarboxylic acids, polycarboxylic anhydrides,and mixtures thereof, wherein component (β) consists of 50 to 100 mol %of aromatic or cycloaliphatic polycarboxylic acids or mixtures thereof,and 15 to 40 mol % of tricarboxylic or tetracarboxylic acids or mixturesthereof, with the caveat that at least two carboxyl groups of thetricarboxylic and tetracarboxylic acids react on statistical average,ina molar ratio of components (α) and (β) of 1.15-2.00:1; and furtherwherein the polyester resin is at least partially neutralized and has anumber average molecular weight of 600 to 5000, an acid value of 20 to70, and a hydroxyl value of 30 to
 200. 18. The paint as claimed in claim12, wherein the component (β) consists of 50 to 80 mol % of aromatic orcycloaliphatic polycarboxylic acids or mixtures thereof, and 20 to 50mol % of one or more polymeric fatty acids, where the polymeric fattyacids are not cycloaliphatic polycarboxylic acids.
 19. The process asclaimed in claim 1, wherein the stage (II) addition is carried out overa period of 10 to 90 minutes.
 20. The paint as claimed in claim 10,wherein the stage (II) addition is carried out over a period of 10 to 90minutes.
 21. A process for the production of a multicoat finish,including the steps of(1) applying an aqueous, pigmented basecoat to asubstrate surface as basecoat, (2) forming a polymer film from thecoating applied in stage (1), (3) applying a transparent topcoat to thebasecoat, and subsequently (4) baking the basecoat together with thetopcoat;wherein the basecoat contains a water-thinnable, uncrosslinkedpolyacrylate resin prepared by a solution polymerization processconsisting essentially of the steps of: (I) polymerizing(a1) 40 to 80%by weight of a (meth)acrylic acid ester essentially free from carboxylgroups selected from the group consisting of alkyl acrylates, alkylmethacrylates, cycloaliphatic acrylic acid esters, cycloaliphaticmethacrylic acid esters, and mixtures thereof, (a2) 4 to 34% by weightof an ethylenically unsaturated monomer which contains at least onehydroxyl group per molecule and is essentially free from carboxylgroups, or a mixture of such monomers, and (a3) 10 to 30% by weight ofan ethylenically unsaturated monomer essentially free from carboxylgroups, which is different from (a1) and (a2), in an organic solvent ormixture of solvents in the presence of at least one polymerizationinitiator, and (II) adding, after at least 80% by weight of the monomersin stage (I) have reacted,(b1) 2.5 to 15% by weight of an ethylenicallyunsaturated monomer containing at least one carboxyl group per molecule,or a mixture of such monomers, and (b2) 0 to 60% by weight of anethylenically unsaturated monomer essentially free from carboxyl groups,or a mixture of such monomers, and continuing the polymerization toobtain the polyacrylate resin, and (III) neutralizing, at leastpartially, the polyacrylate resin and dispersing it in water;wherein thesum of (a1), (a2), (a3), (b1) and (b2) is 100% and further wherein theuncrosslinked polyacrylate resin has a hydroxyl value of 0 to 200, anacid value of 20 to 100, a glass transition temperature (T_(G)) of -40°to +60° C., and a molecular weight of between 2,500 and 20,000.
 22. Anaqueous, pigmented paint containing a water-thinnable polyacrylate resinprepared by a solution polymerization process consisting essentially ofthe steps of:(I) polymerizing(a1) 40 to 80% by weight of a (meth)acrylicacid ester essentially free from carboxyl groups selected from the groupconsisting of alkyl acrylates, alkyl methacrylates, cycloaliphaticacrylic acid esters, cycloaliphatic methacrylic acid esters, andmixtures thereof, (a2) 4 to 34% by weight of an ethylenicallyunsaturated monomer which contains at least one hydroxyl group permolecule and is essentially free from carboxyl groups, or a mixture ofsuch monomers, and (a3) 10 to 30% by weight of an ethylenicallyunsaturated monomer essentially free from carboxyl groups, which isdifferent from (a1) and (a2), or a mixture of such monomers, in anorganic solvent or mixture of solvents in the presence of at least onepolymerization initiator, and (II) adding, after at least 80% by weightof the monomers in stage (I) have reacted,(b1) 2.5 to 15% by weight ofan ethylenically unsaturated monomer containing at least one carboxylgroup per molecule, or a mixture of such monomers, and (b2) 0 to 60% byweight of an ethylenically unsaturated monomer essentially free fromcarboxyl groups, or a mixture of such monomers, and continuing thepolymerization to obtain the polyacrylate resin, and (III) neutralizing,at least partially, the polyacrylate resin and dispersing it inwater;wherein the sum of (a1), (a2), (a3), (b1), and (b2) is 100% andfurther wherein the uncrosslinked polyacrylate resin has a hydroxylvalue of 0 to 200, an acid value of 20 to 100, a glass transitiontemperature (T_(G)) of -40° to +60° C., and a molecular weight ofbetween 2,500 and 20,000.
 23. A process for the production of amulticoat finish according to claim 1, wherein the uncrosslinkedpolyacrylate resin has a hydroxyl value of 20 to 120, an acid value of25 to 50, and a glass transition temperature (T_(G)) of -20° to +40° C.24. The process according to claim 16, wherein the mixture consistsof:(A) 25 to 70% by weight of the water-thinnable polyacrylate resin,(B) 10 to 40% by weight of the amino resin, (C) 20 to 60% by weight ofthe water-thinnable polyester resin, and (D) 0 to 40% by weight of thewater-thinnable polyurethane resin.
 25. The process according to claim7, wherein the mixture consists of:(A) 1 to 30% by weight of thewater-thinnable polyacrylate resin, (B) 5 to 30% by weight of the aminoresin, (C) 15 to 40% by weight of the water-thinnable polyester resin,and (D) 20 to 60% by weight of the water-thinnable polyurethane resin.26. The process as claimed in claim 8, wherein the molar ratio ofcomponents (α) and (β) is 1.2-1.5:1, and further wherein the polyesterresin has a number average molecular weight of 800 to 2500, an acidvalue of 25 to 55, and a hydroxyl value of 45 to
 100. 27. The paintaccording to claim 15, wherein the mixture consists of:(A) 25 to 70% byweight of the water-thinnable polyacrylate resin, (B) 10 to 40% byweight of the amino resin, (C) 20 to 60% by weight of thewater-thinnable polyester resin, and (D) 0 to 40% by weight of thewater-thinnable polyurethane resin.
 28. The paint according to claim 16,wherein the mixture consists of:(A) 1 to 30% by weight of thewater-thinnable polyacrylate resin, (B) 5 to 30% by weight of the aminoresin, (C) 15 to 40% by weight of the water-thinnable polyester resin,and (D) 20 to 60% by weight of the water-thinnable polyurethane resin.29. The paint as claimed in claim 16, wherein the molar ratio ofcomponents (α) and (β) is 1.2-1.5:1, and further wherein the polyesterresin has a number average molecular weight of 800 to 2500, an acidvalue of 25 to 55, and a hydroxyl value of 45 to 100.